1. Field of the Invention
This invention relates to an improved process for obtaining molds for metal castings, especially molds of large dimension. This invention is directed to a process for making a mold which not only can be employed in making metal castings of large dimensions, but can be employed in making metal castings of high dimensional accuracy. This invention is directed to, additionally, a ceramic composition for use in making molds for metal castings which ceramic composition comprises a binding agent, finely divided refractory material anti-setting and thixotropic agents and special solvents.
2. Discussion of the Prior Art
Numerous attempts have been made to prepared molds for metal castings. In German Pat. No. 942,340 there is described a process for making molds wherein crushed refractory substance are used together with a binder such as one based on an alkyl silicate. Molds made in accordance with the process therein described, however, do not satisfy requirements with respect to dimensional accuracy. Additionally, such molds are only suitable for making castings of small dimensions. It has, therefore, become desirable to provide a method for making molds to be employed in the manufacture of metal castings of high dimensional accuracy and of large dimensions.
There is described another process for the manufacture of molds in Foundry Trade Journal of Mar. 3, 1960 under the title "Composite Mould Process". In the process therein disclosed two different ceramic molding materials are used: grog with a water glass binder for making a supporting mold and a fluid ceramic slip consisting of a crushed refractory material, a binding agent and a hardener. The grog mold is initially made on a preliminary pattern which is approximately 1 to 2 cm. larger in all dimensions than the actual pattern. The grog mold is fired at 650.degree. C. The same is provided with a hole in the center and it is placed over the actual pattern and the slip is poured through the hole into the clearance between the pattern and the preliminary form. The cast layer constitutes the lining which comes in contact with the molten metal during casting. The disadvantage of this process lies in the occurrance of air bubbles in the poured slip often resulting in surface flaws. Obviously, several operations are required in the manufacture of the mold, i.e., the formation of a grog mold, the disposition of the grog mold over the actual pattern and the filling of the clearance between the grog mold and the actual pattern, all consuming time and money.
To eliminate the disadvantages of that process it was proposed in German Offenlegungsschrift No. 1,965,446 first to coat the pattern with a thin fine coating and then to sprinkle coarse granular material on the coating while it was still wet, i.e., before it had fully set and then to draw a preliminary form over the coated pattern. The preliminary form would, of course, be 1 to 2 cm. larger in all dimensions than the pattern. A ceramic slip would then be poured into the clearance. Since in the practice of this process at least four operations are necessary it consumed too much time and effort. It was also impractical for the manufacture of metal castings of large dimensions.
In the periodical, "Giesseri", 50, p. 661 (1963), a ceramic molding process is described in which a divided pattern is repeatedly dipped into a ceramic slip. In this manner a thin-walled ceramic shell mold is formed on the pattern. The process is, however, limited to the production of molds of relatively small dimensions. It is not useful for the manufacture of castings having steep descending walls.
In the periodical "Gray and Ductile Iron News" of October 1966 there is described another process for the manufacture of molds wherein colloidal silica is employed as binding agent. A ceramic shell is formed which is provided with a supporting shell. Unfortunately, this process also is limited to the preparation of molds of small dimensions.
Hence, none of the methods thus far provided have been simple in nature and yet able to prepare molds for the manufacture of metal castings of large dimensions, especially those having steeply descending outer walls, say, those having a pitch of at least 90.degree.. Attempts have, indeed, been made to use sprayable compositions before the ceramic coating comes in contact with the metal and to thereafter sand these compositions. Such sand compositions, however, have proven to be impractical for the preparation of ceramic coatings on patterns with vertical or steep outer walls, especially outer walls having a pitch of at least 90.degree.. On account of their relatively low viscosity they run down the steep walls and become accumulated at the bottom. The viscosity can be increased by increasing the percentage of the highly refractory substances, but this does not solve the problems because even such more viscous materials tend to sag and run down the walls.
It, therefore, became desirable to provide a process for the manufacture of a mold suitable for metal castings which could be simply performed, could prepare molds having large dimensions and would be useful particularly in the preparation of castings having vertically disposed or steeply descending outer walls, especially those having a pitch of at least 90.degree.. It became especially desirable to provide a process for the manufacture of a mold useful in metal casting whereby the mold would provide great dimensional accuracy for the metal casting.